A molecular sieve is generally a microporous structure composed of either crystalline aluminosilicate, chemically similar to clays and feldspars and belonging to a class of materials known as zeolites, or crystalline aluminophosphates derived from mixtures containing an organic amine or quaternary ammonium salt, or crystalline silicoaluminophosphates which are made by hydrothermal crystallization from a reaction mixture comprising reactive sources of silica, alumina and phosphate. Molecular sieves have a variety of uses. They can be used to dry gases and liquids; for selective molecular separation based on size and polar properties; as ion-exchangers; as catalysts in cracking, hydrocracking, disproportionation, alkylation, isomerization, oxidation, and conversion of oxygenates to hydrocarbons; as chemical carriers; in gas chromatography; and in the petroleum industry to remove normal paraffins from distillates.
Molecular sieves are manufactured by reacting a mixture of several chemical components. One of the components used in the reaction process is a template, although more than one template can be used. The templates are used to form channels or tunnel like structures (also called a microporous structure) within the composition. When the template is removed, an open microporous structure is left behind in which chemical compositions can enter, as long as the chemical compositions are small enough to be able to fit inside the tunnels. Thus a molecular sieve acts to sieve or screen out large molecules from entering a molecular pore structure.
Molecular sieves are particularly desirable for use as catalytic agents. The molecular sieves that act as catalysts have catalytic sites within their microporous structures. Once the template is removed, a chemical feedstock that is small enough to enter into the tunnels can come into contact with a catalytic site, react to form a product, and the product can leave the molecular sieve through any number of the tunnels or pores as long as the product has not become too large to pass through the structure. The pore sizes typically range from around 2 to 10 angstroms in many catalytic molecular sieves.
Template material can be removed from the framework of a molecular sieve by a variety of methods. A preferred method, however, is by calcining or heat treating in an oxygen environment, since calcining under appropriate conditions brings the additional advantage of hardening the molecular sieve. Once the molecular sieve is hardened, it can be more readily transported or more effectively blended with other materials.
U.S. Pat. No. 5,174,976 discloses one method of calcining a molecular sieve material in order to remove the template material. The method includes the steps of heating a crystalline [metallo]aluminophosphate composition to a calcination temperature at a rate no greater than 10° C. per minute with a high flow rate of a non-oxidizing gas, e.g., nitrogen, and thereafter with an oxygen-containing gas, e.g. air, at high gas flow rates, e.g., 100 to 400 cc/minute/gram. Calcination temperature is described as ranging from 100-600° C.
In U.S. Pat. No. 4,681,864, it is disclosed, however, that calcination of SAPO-37 molecular sieve compositions to remove the template material leaves a structure which is sensitive to contact with moisture. A method of removing template in order to avoid the moisture problem is suggested. Specifically, the method involves preparing a SAPO-37 molecular sieve with a template, and leaving the entire template in place for storage purposes. The molecular sieve is stated to contain an organic template in its pore structure in amounts ranging from 1 to 50% by weight of the molecular sieve, with an inorganic oxide matrix component such as silica, alumina, silica-alumina gels and sols, clay, and mixtures thereof. The entire template is removed by placing the molecular sieve in a catalytic cracking unit at 400-600° C.
Methods which are conventionally used to remove template material either fail to provide adequate protection against contact with moisture or fail to sufficiently harden the catalyst material so that it can be transported from one location to another with little physical damage. In general, it even appears that moisture damage is not a generally recognized problem. This is suggested, for example, by Hawley's Condensed Chemical Dictionary, Thirteenth Edition, Von Nostrand Reinhold, 1997, where it is stated that one characteristic of the molecular sieve materials is their ability to undergo dehydration with little or no change in crystalline structure. Nevertheless, even the few methods that have been suggested for providing protection of specific molecular sieve compositions do not provide a product that would be hard enough to withstand many of the physical tortures encountered during transportation, much less the physical tortures that would be encountered during actual use. Therefore, there is a need to provide molecular sieves that are effectively protected from damage due to contact with moisture and from damage due to physical contact.